The present invention relates to polymeric linings for protecting the surface of a substrate from contamination or corrosion by a fluid.
It is known in the art to provide linings adjacent the surface of a substrate to protect the substrate from contamination or corrosion. Protective linings are well known in the chemical process industry for protecting equipment such as vessels, pipes, pumps and valves from corrosive fluids handled by the equipment. Components of such equipment to be protected are commonly made from steel because of strength, ease of manufacture and cost considerations but may also be made from other materials.
Early forms of protective linings included linings made from glass for protecting inner walls of steel vessels. The protection provided by these glass linings greatly extended the service life of processing equipment in certain applications. However, glass linings are not resistant to certain fluids frequently handled or generated by chemical process equipment. For example, glass will be dissolved by contact with hydrofluoric acid.
More recently, linings made from polymers have been used for protecting substrates, including components of chemical process equipment. Prominent among the polymers used for protection of process equipment are various fully (i.e., perfluorinated) and partially fluorinated polymers. Linings made from fully fluorinated polymers have advantages over other materials such as glass in certain applications because they are chemically inert and highly resistant to chemical attack. Fully fluorinated polymers which have been used in this capacity include polytetrafluoroethylene (PTFE), and perfluoroalkoxy-tetrafluoroethylene copolymer (PFA), fluorinated ethylene-propylene copolymer (FEP). PFA is a copolymer of tetrafluoroethylene [CF2=CF2] with a perfluoralkyl vinyl ether [F(CF2)mCF2OCFxe2x95x90CF2]. The resultant polymer contains the carbon-fluorine backbone chain typical of PTFE with perfluoroalkoxy side chains. One particular form of PFA which has been used for linings is tetrafluoroethylene-perfluoromethylvinylether copolymer (MFA). Partially fluorinated polymers such as ethylene-chlorotrifluoroethylene copolymer (ECTFE), ethylene-tetrafluoroethylene copolymer (ETFE) and polyvinylidene fluoride (PVDF) have also been used in linings for chemical process equipment although these materials are less chemically resistant than the fully fluorinated polymers.
Known methods of applying polymeric linings to a substrate include coating processes in which a polymeric material is applied to the substrate in the form of a liquid dispersion or a powder. Liquid dispersions, comprising polymeric material in a water or solvent suspension, are typically applied in a spray and bake coating process in which the liquid dispersion is sprayed onto the substrate for subsequent heating above the melting temperature of the polymeric material contained in the dispersion. Known methods of applying polymeric material in powdered form include spraying of the powder onto the substrate using an electrostatic gun or a flocking gun, for example. Typically, the powder is sprayed onto a substrate which has been heated above the melt temperature of the polymeric material to form a lining. It is also known to apply linings in a process known as xe2x80x9crotoliningxe2x80x9d in which the substrate and powder is heated, in an oven for example, above the melt temperature of the polymeric material while the substrate is rotated to form a seamless lining on the substrate.
Early efforts to deal with issues relating to bonding of the polymer lining to the substrate included chemical bonding systems in which a primer was applied to the substrate prior to coating with polymer. More recently, mechanical bonding systems have been used to augment or replace the chemical bonding provided by the primer process. An example of a mechanical bonding system includes the use of wire mesh secured to the substrate prior to application of the coating to form the polymer lining. The wire mesh is typically secured to the substrate by tack welding, or xe2x80x9cmicro-weldingxe2x80x9d, to achieve intermittent fusion of the mesh to the substrate without affecting the integrity of the substrate. In a known method, a resistance welding tool is directed over the mesh covered substrate to create fusion between the mesh and the substrate as the welding tool contacts the threads of the mesh. The polymer lining is then applied to the mesh-covered substrate in a polymer coating process wherein melt flow of a polymer contained in a liquid dispersion or powder results in intermingling interaction between the threads of the mesh and the lining to interlock the lining and the mesh. The location of the mesh in flush contact with the surface of the substrate results in contact between the polymer and the substrate surface between the threads of the mesh. The mechanical interlocking of the lining to the mesh results in enhanced performance and extended life for chemical process equipment incorporating this system.
The polymeric linings of the prior art systems provide barrier protection for substrates such as reaction and storage vessels by reducing contact of the substrate with corrosive liquids and gases. However, polymer linings are not impervious to propagation of corrosive fluids which may eventually penetrate the lining and become trapped on the substrate side of the lining, in contact with the substrate. This is particularly true for chemical process equipment which may operate at elevated temperatures and pressures. The rate of propagation of a fluid through a polymer lining is generally enhanced under increased temperatures and pressures. The propagation of corrosive fluid through the protective lining to the backside of the lining is undesirable not only for reasons of contamination or corrosion of the substrate surface but also for reasons of degradation of the chemical and/or mechanical bonding system securing the lining to the substrate. The present invention provides for the channeling of the propagated fluid at the backside of the lining facilitating removal of the fluid in a venting removal system for example.
U.S. Pat. No. 4,166,536 to Roberts et al. is directed to a corrosive chemical containment system. A vessel contains a barrier coating of a corrosion-resistant polymer matrix which includes an inert filler. The coating has a controlled porosity. According to Roberts et al., chemicals trapped in the coating can permeate, so as to prevent delamination, blistering rupture or other types of coating failures.
What is needed is a polymeric lining system for protecting a substrate such as a vessel such that fluids which have propagated through the lining to the interface between the lining and the substrate are channeled away.
According to the present invention there is provided a substrate protected by a polymeric lining positioned adjacent a surface of the substrate. Passages are provided between the lining and the substrate for channeling of fluids which may penetrate through the lining to the substrate surface.
According to an embodiment of the invention, the lining is formed to provide for passages between the lining and the substrate. Structure attached to or formed in the substrate provides for securement of the lining to the substrate. Preferably, the structure is a mesh which is secured to the substrate surface and which becomes at least partially embedded in the lining to secure the lining to the mesh. Elongated spacers are positioned between the mesh and the substrate and function to separate portions of the lining from the substrate surface such that the separated portions of the lining and the substrate define passageways for channeling fluid which may penetrate the lining. Preferably, the elongated spacers are lengths of wire having a circular cross-section.
According to an embodiment of the invention, the substrate surface is formed to provide for passages between the lining and the substrate.
According to an embodiment of the invention, a surface of a vessel is protected by a lining wherein either or both of the vessel surface and the lining are formed or disposed to provide passageways for channeling fluid which may penetrate through the lining. A mesh is secured to the vessel surface over a network of spacers which include spacers extending substantially parallel to one another to form a generally parallel array of passageways and at least one spacer which is transverse to the parallel array of passageways to form a generally transverse passageway.
According to an embodiment of the invention the lining is made from a fluoropolymer selected from the group consisting of polytetrafluoroethylene, perfluoroalkoxy-tetrafluoroethylene copolymer, fluorinated ethylene-propylene copolymer, ethylene-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride and combinations thereof.
The foregoing and other features and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments thereof, as illustrated in the accompanying figures.